This invention relates to semiconductor memory devices.
More particularly, the present invention relates to semiconductor random access memory devices that utilize a magnetic field.
Memory devices are an extremely important component in electronic systems. The three most important commercial high-density memory technologies are SRAM, DRAM, and FLASH. Each of these memory devices uses an electronic charge to store information and each has its own advantages. SRAM has fast read and write speeds, but it is volatile and requires large cell area. DRAM has high density, but it is also volatile and requires a refresh of the storage capacitor every few milliseconds. This requirement increases the complexity of the control electronics.
FLASH is the major nonvolatile memory device in use today. Typical non-volatile memory devices use charges trapped in a floating oxide layer to store information. Drawbacks to FLASH include high voltage requirements and slow program and erase times. Also, FLASH memory has a poor write endurance of 104-106 cycles before memory failure. In addition, to maintain reasonable data retention, the thickness of the gate oxide has to stay above the threshold that allows electron tunneling, thus restricting FLASH""s scaling trends.
To overcome these shortcomings, magnetic memory devices are being evaluated. One such device is magnetoresistive random access memory (hereinafter referred to as xe2x80x9cMRAMxe2x80x9d). MRAM has the potential to have speed performance similar to DRAM. To be commercially viable, however, MRAM must have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds.
For an MRAM device, the stability of the memory state, the repeatability of the read/write cycles, and the power consumption are some of the most important aspects of its design characteristics. A memory state in MRAM is not maintained by power, but rather by the direction of the magnetic moment vector. Storing data is accomplished by applying magnetic fields and causing a magnetic material in a cell to be magnetized into either of two possible memory states. Recalling data is accomplished by sensing resistive changes in the cell when magnetic fields are applied. The magnetic fields are created by passing currents through strip lines external to the magnetic structure or through the magnetic structures themselves.
Conventional MRAM devices rely on the bit shape with aspect ratio to create a shape anisotropy that provides the switching field. As the bit dimension shrinks, three problems occur. First, the switching field increases for a given shape and film thickness, requiring more current to switch. Second, the total switching volume is reduced so that the energy barrier for reversal, which is proportional to volume and switching field, decreases. The energy barrier refers to the amount of energy needed to switch the magnetic moment vector from one state to the other.
The switching field is also dependant on material properties, so there is a lower limit for typical materials. As the switching field increases, more current is required to change the state of a MRAM bit and, consequently, more power is consumed. Therefore, a need exists to lower the switching field and decrease the power consumption of MRAM devices.
Accordingly, it is an object of the present invention to provide a new and improved magnetoresistive random access memory device with a decreased switching field.
To achieve the objects and advantages specified above and others, a magnetoresistive tunneling junction memory cell with a switching field and an easy axis is disclosed. The MRAM cell includes a first magnetic region, the first magnetic region has a resultant magnetic moment vector fixed in a preferred direction in the absence of an applied magnetic field wherein the first magnetic region has a magnetic fringing field. An electrically insulating material is positioned on the first magnetic region to form a magnetoresistive tunneling junction.
A second magnetic region is positioned on the electrically insulating material and has a magnetic moment vector adjacent to the tunnel barrier and oriented in a direction parallel or anti-parallel to that of the first magnetic region. In the preferred embodiment, at least one of the first and second magnetic regions include N ferromagnetic layers which are anti-ferromagnetically coupled, where N is a whole number greater than or equal to two. Further, the memory cell operates in a toggle writing mode wherein the magnetic fringing field creates a bias field within the second magnetic region and along the easy axis and, consequently, changes the switching field of the magnetoresistive tunneling junction memory cell.